The present invention generally relates to a method for forming an LCD package and device formed and more particularly, relates to a method for forming a compact LCD package that requires a substantially smaller planar area on the LCD for making connections to a driver chip and an external circuit and device formed.
In recent years, liquid crystal display (LCD) panels have been used widely in place of cathode ray tubes (CRT) in electronic display applications. The LCD panel is first assembled together by filling a liquid crystal material in between a LCD substrate and a transparent glass cover plate. The LCD substrate consists of a multiplicity of switching, or electronic turn-on and turn-off devices for operating the multiplicity of pixels formed on the LCD panel.
After the assembly of a LCD panel is completed, the panel must be connected to an outside circuit for receiving electronic signals such that images may be produced in the panel. The electronic connections to the LCD panel can be provided by different techniques. A conventional technique for providing electronic signals to a LCD panel is by using a flexible printed circuit board (FPCB) which contains various electronic components welded thereto for providing signals to the LCD. The flexible printed circuit board is frequently manufactured of a conductive copper layer sandwiched between two flexible polyimide cover layers. The flexibility of FPCB is advantageous in the installation of a LCD panel. FIG. 1A is a graph illustrating a conventional design of LCD panel 10 connected by a FPCB 12 to a printed circuit board (PCB) 14 which has a surface mount technology (SMT) type IC chip 16 mounted on top.
In another conventional technique for bonding a LCD to a PCB, as shown in FIG. 1B, a tape automated bonding (TAB) technique is used. In the TAB bonding technique, a TAB section 20 is used to connect LCD 10 and PCB 14. The TAB section 20 consists of a TAB tape 22 which has an IC chip 16 connected to it through bonding sites 24. The TAB bonding technique provides the benefit of a compact package so that circuit density can be improved resulting in a lead pitch as low as 60 xcexcm. The TAB, also known as TCP (tape carrier package) utilizes finely patterned thin metal, i.e., copper foil plated with Au or Sn, in place of wires and connects the metal tips metallurgically to corresponding gold plated bumps that are formed on the aluminum pads on the chip. TAB is preferred in smaller-pitch interconnects for high I/O ULSI devices because it enables smaller pitch and longer span bondings than those available by wire bonding. However, the TAB bonding technique is normally conducted at a higher fabrication cost.
In still another technique for bonding a LCD to a PCB, as shown in FIG. IC, a chip on glass (COG) technique is used. In the COG technique, an IC chip 16 is mounted directly on a LCD 10 by utilizing solder bumps 24 and an anisotropic conductive film (ACF) 26. Detailed cross-sectional views of an ACF 26 is shown in FIGS. 2A and 2B. As shown in FIG. 2A, a TAB tape 22 which has conductive pads 28 formed on top is positioned over an ACF tape 30 which contains electrically conductive particles 32 embedded an insulative compound 34. Positioned under the ACF 30 is a LCD substrate 10 which has conductive elements 36 formed on top. After the TAB tape 22, the ACF 30 and the LCD substrate 10 are pressed together under heat, as shown in FIG. 2B, the conductive particles 32 provides electrical communication between the conductive pads 28 and the conductive elements 36 and therefore allowing the TAB tape 22 to electrically communicate with the LCD substrate 10. It should be noted that, electrical communication between the conductive pads 28 and the conductive elements 26 is only established where the conductive particles 32 are compressed, i.e., only established anisotropically and selectively. The conductive elements 36 on the LCD substrate 10 is normally formed of indium-tin-oxide (ITO) thin films.
As shown in FIG. 1C, the COG technique may further connect the LCD substrate 10 to a printed circuit board (not shown) by a flexible printed circuit board (not shown). The COG technique therefore relies on bonding with solder bumps 24 formed on an IC chip and the ACF for electrical communications.
Each of the three conventional techniques for forming a TFT-LCD assembly has its benefits and disadvantages. For instance, in the first technique of using SMT/FPCB, the circuit density can be increased to achieve a compact package at the expense of using difficult TAB technology and high material costs. In the TAB and COG method, a rework of the assembly such as the removal of a defective IC from a LCD substrate is extremely difficult, if not impossible. For instance, the only possible means for removing an IC chip that is bonded to a LCD substrate is by using a shear force for pushing an IC chip and breaking its bond with the LCD substrate. This is a difficult process and frequently results in the destruction of the entire assembly.
In the present fabrication process for TFT-LCD assemblies, the SMT/FPCB method is frequently used in fabricating lower priced assemblies such as those utilizing small LCD panels. In large LCD panel applications, i.e., such as those used in notebook computers, the TAB bonding method is normally used. The COG method, due to its difficulty in reworking and repair, is also limited to small LCD panel display applications. The TAB process and the COG process are therefore the two major assembling methods used for TFT-LCD assemblies. To sum up, the TAB method can be easily reworked and repaired by removing an IC chip from the TAB tape and furthermore, it is compact in size which allows the achievement of high density packages of up to 60 xcexcm pitch. However, the TAB process requires complicated fabrication steps which include IC bonding, tape fabrication, inner lead bonding, encapsulation, outer lead bonding and the ACF process. Another drawback for the TAB process is, during the ACF processing, there is a thermal expansion problem which may lead to misalignment between the leads. The TAB tape may further absorb moisture and contribute to its dimensional instability. Elaborate equipment may also be required for the TAB process and therefore increasing its fabrication costs.
In the COG process, while the fabrication steps required are simpler, i.e., only IC bumping and ACF processes are required, and furthermore, there is no thermal expansion problem and smaller pitch such as 50 xcexcm can be achieved, the fabrication of an LCD package of compact size is nevertheless difficult. This difficulty is illustrated in FIGS. 3A and 3B. In FIG. 3A, a cross-sectional view of an LCD package 40 which consists of an LCD substrate shown as a lower glass panel 10, an upper glass panel 42, an IC chip (or a driver chip) 16, an anisotropic conductive film (ACF) layer 26, and a printed circuit board (or a flexible printed circuit board) 14 is shown. The driver chip 16 is equipped with a multiplicity of solder bumps 44 formed on an active surface of the chip 16 for making electrical connection with a second multiplicity of conductive leads 46 (such as ITO leads) formed on the top surface 48 of the lower glass panel 10. The electrical communication is established by using the anisotropic conductive film layer 26 which is loaded with electrically conductive particles 50. In order to communicate with the outside circuits, such as circuits on the PCB 14, electrical communication through a third multiplicity of conductive pads 52 is provided between the PCB 14 and the lower glass panel 10. A plane view of the LCD package 40 is further shown in FIG. 3B.
As shown in FIGS. 3A and 3B, the lower glass panel 10 of the LCD package 40 is normally provided with a larger area than the upper glass panel 42 and thus leaving an exposed edge area 60 necessary for mounting of the driver chip 16 and the PCB 14. The excess edge portion 60 therefore cannot be reduced which results in excessively large LCD packages that are not suitable for certain applications that require a compact LCD package. The technology of connecting driver chips and external PCB circuits to an LCD package therefore must be improved in order to produce compact LCD panels.
It is therefore an object of the present invention to provide a method for forming a compact LCD package that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for forming a compact LCD package by stacking a driver chip on top of a printed circuit board prior to assembling the board to the LCD panel.
It is a further object of the present invention to provide a method for forming a compact LCD package that does not require separate planar areas for mounting a driver chip and a PCB.
It is another further object of the present invention to provide a method for forming a compact LCD package by first bonding a PCB to a lower glass panel of an LCD device.
It is still another object of the present invention to provide a method for forming a compact LCD package by first bonding a flexible PCB to the lower glass panel of a LCD by using an anisotropic conductive film layer.
It is yet another object of the present invention to provide a method for forming a compact LCD package by first bonding a PCB to the lower glass panel of a LCD and then bonding a driver chip on top of the PCB by solder bumps.
It is still another further object of the present invention to provide a compact LCD package that includes an LCD panel which has an exposed edge portion formed by a lower glass panel, a printed circuit board bonded on top of the exposed edge portion of the LCD panel, and a driver chip bonded to the top of the PCB.
It is yet another further object of the present invention to provide a compact LCD package that consists of an LCD panel that has an exposed edge portion formed by a lower glass panel, a flexible printed circuit board bonded to the exposed edge portion by an anisotropic conductive film layer, and a driver chip bonded to the top of the PCB by solder bumps.
In accordance with the present invention, a method for forming a compact LCD package and a device formed by the method are disclosed.
In a preferred embodiment, a method for forming a compact LCD package can be carried out by the operating steps of first providing an LCD panel by bonding an upper glass panel and a lower glass panel together with a liquid crystal material sandwiched thereinbetween, the lower glass panel is larger in area than the upper glass panel providing an exposed edge portion which is equipped with a first multiplicity of conductive elements thereon, then providing at least one driver chip equipped with solder bumps on an active surface for electrically connecting to the first multiplicity of conductive elements on the lower glass panel, providing a printed circuit board which has a second multiplicity of conductive leads on a top surface in electrical communication with a third multiplicity of conductive leads on a bottom surface, providing an insulating film of thermally fusible material that is loaded with electrically conductive particles therein, positioning sequentially the insulating film of thermally fusible material on top of the exposed portion of the lower glass panel over the first multiplicity of conductive elements, and the PCB on top of the insulating film with the third multiplicity of conductive leads contacting the film forming a first stack, compressing the first stack under a pressure and temperature sufficient to establish electrical communication between the third multiplicity of conductive leads on the PCB and the first multiplicity of conductive elements on the exposed edge portion of the LCD panel through the electrically conductive particles in the insulating film, positioning the at least one driver chip on top of the PCB with the solder bumps contacting the second multiplicity of conductive leads forming a second stack, and compressing the second stack under a pressure and temperature sufficient to establish electrical communication between the solder bumps on the at least one driver chip and the second multiplicity of conductive leads on the PCB.
In the method for forming a compact LCD package, the first stack formed by the PCB positioned on the exposed edge portion of the LCD display panel occupies a substantially reduced planar area. The first multiplicity of conductive elements is adapted for connecting to drive lines and scan lines of the LCD display panel. The at least one driver chip may be at least one flip chip. The method may further include the step of providing a flexible PCB which has a second and a third multiplicity of conductive leads thereon. The method may further include the step of providing the insulating film in an anisotropic conductive film. The method may further include the step of providing the insulating film in a polymeric based film filled with electrically conductive particles.
In the method for forming a compact LCD package, the method may further include the step of forming the first multiplicity of conductive leads with indium-tin-oxide (ITO) traces on a glass surface. The method may further include the step of forming the second multiplicity and the third multiplicity of conductive leads with an electrically conductive metal selected from the group consisting of copper and aluminum, or with copper coated with gold. The method may further include the step of forming solder bumps on the at least one driver chip in gold. The substantially reduced planar area may be about 50% of the original area required for connecting the PCB.
The present invention is further directed to a compact LCD package that includes an LCD panel formed of an upper glass panel and a lower glass panel, the lower glass panel is larger in area than the upper glass panel for producing an exposed edge portion equipped with a first multiplicity of conductive leads, a printed circuit board which has a third multiplicity of conductive leads on a bottom surface bonded to the first multiplicity of conductive leads on the exposed edge portion, and at least one drive chip equipped with a fourth multiplicity of solder bumps on at least one active surface bonded to a second multiplicity of conductive leads on a top surface of the PCB.
The compact LCD package may further include an insulating film filled with electrically conductive particles sandwiched between the PCB and the exposed edge portion of the LCD lower glass panel, the conductive particles provide electrical communication between the third multiplicity of conductive leads on the PCB and the first multiplicity of conductive leads on the. LCD panel. The first multiplicity of conductive leads on the LCD lower glass panel are drive lines and scan lines for the LCD. The package may further include an anisotropic conductive film layer sandwiched between the PCB and the LCD panel. The at least one driver chip may be at least one flip chip.
In an alternate embodiment, the present invention discloses a two-step method for bonding a driver chip to an LCD panel which includes the steps of pre-bonding a printed circuit board to an edge portion of an LCD panel by electrically connecting a second multiplicity of conductive leads on a bottom surface of the PCB to a first multiplicity of conductive leads on the edge portion of the LCD panel, and post-bonding a driver chip to the PCB by electrically connecting solder bumps formed on a bottom surface of the drive chip to a third multiplicity of conductive leads formed on a top surface of the PCB.
The two-step method for bonding a driver chip to an LCD panel may further include the step of pre-bonding the PCB to the LCD panel by using an anisotropic conductive film sandwiched thereinbetween. The pre-bonding step may be conducted by an outer lead bonding technique and the post-bonding step may be conducted by an inner lead bonding technique. The method may further include the step of providing a flexible printed circuit board for the PCB. The method may further include the step of providing the first multiplicity of conductive leads on the edge portion of the LCD panel for connecting to drive lines and scan lines of the LCD panel.